1. Field of the Invention
The present invention relates to a multi-beam scanning apparatus, and particularly to a scanning apparatus which is suitably applicable to image forming apparatuses, such as laser beam printers, digital copying machines, and multi-function printers that employ the electrophotographic process, and in which a light beam from a light source unit is reflectively deflected by a polygon mirror serving as a deflecting unit, transmitted through a scanning optical system, and scanned on a surface to be scanned (a scanned surface) to record image information. More particularly, the present invention relates to a multi-beam scanning apparatus in which plural light beams are simultaneously scanned to achieve operation with high speed and fine precision, and a deflecting unit, such as a polygon mirror, is effectively used to downsize the overall apparatus.
2. Related Background Art
FIG. 21 schematically illustrates a main portion of a conventional multi-beam scanning apparatus.
In FIG. 21, two beams emitted from a light source unit 91 and optically modulated according to image information are converted into approximately parallel light beams by a collimator lens 92, and are incident on a cylindrical lens 93. Each light beam incident on the cylindrical lens 93 emerges therefrom without any change in a main scanning section, and passes through an aperture stop 94. The light beam is partially intercepted by the aperture stop 94. With respect to a sub scanning section, each beam is converged by the cylindrical lens 93, passes through the aperture stop 94 with a portion of the light beam being intercepted, and is imaged on a deflecting facet 95a of a deflecting unit 95 as an approximately linear image (a linear image extending in a main scanning direction). Each light beam reflectively deflected by the deflecting facet 95a of the deflecting unit 95 is imaged on a photosensitive drum surface 97 in the form of a spot by a scanning optical system (a scanning lens) 96. The photosensitive drum surface 97 is scanned with the imaged spot moving at a uniform speed in a direction of an arrow B (the main scanning direction) when the deflecting unit 95 is rotated in a direction of an arrow A. Accordingly, two scanning lines are simultaneously formed on the photosensitive drum surface 97 serving as a recording material such that image recording can be executed.
At the time of the above-discussed operation, a portion (a BD light beam) of each light beam reflectively deflected by the deflecting unit 95 is guided to a light detecting device (a BD sensor) 99 of a synchronous detecting unit through a folding mirror (a BD mirror) 98 of the synchronous detecting unit by the scanning optical system 96, so that timing of a scanning start position on the photosensitive drum surface 97 can be adjusted prior to the scanning of the photosensitive drum surface 97 with the light spot. For each BD light beam, the timing of the scanning start position for image recording on the photosensitive drum surface 97 is adjusted, using a synchronous signal (a BD signal) obtained by detecting an output signal from the BD sensor 99. In FIG. 21, only a principal ray of each light beam is illustrated for the convenience of simplicity and easy understanding.
In such a multi-beam scanning apparatus, in order to obtain a high-quality image, the radiation amount of each light beam is adjusted (auto-power control (APC)) such that the amount of light projected on the photosensitive drum surface can be stably made constant, prior to the start of writing scanning lines.
Further, there has been proposed a multi-beam scanning apparatus which includes a light-amount monitor for performing synchronous detection (BD detection) and adjustment (APC) of the light amounts of plural light beams emitted from a light source unit, and in which the synchronous detection and the light-amount adjustment of the plural light beams are sequentially executed.
Those conventional multi-beam scanning apparatuses, however, do not mention the light radiation order of the light source unit by any means. Accordingly, the deflecting unit (the polygon mirror) therein cannot be effectively used, and hence there is a need to use a large deflecting unit.
Further, in a case where ends of the deflecting facet in the deflecting unit are chamfered, a light beam incident on the chamfered portion is likely to reach the photosensitive drum surface, leading to occurrence of the problem of ghost.